In many vehicular applications it is desirable to control the power applied to the drive train in a manner that reduces or prevents slip of the vehicle's wheels. Slip typically occurs when the torque applied to the wheels of the vehicle is larger than the maximum friction force between the wheels and the surface on which the wheels are riding. In a conventional locomotive, power control is provided by a throttle manually set by a locomotive operator. The throttle command is converted into electric current supplied to a plurality of electric traction motors. An alternator and rectifier bridge may supply direct current (DC) to a traction inverter, which may in turn provide controlled alternating current (AC) to one or more AC electric induction motors used as the traction motors for powering the wheels or other traction components of the vehicle. AC induction motors are widely used in automotive and industrial applications due, in large part, to their low cost, reliability, ruggedness, and simplicity. They typically consist of a stator and a rotor. The stator is a stationary member, and the rotor is a rotatable member positioned on a shaft within the stator. Coils are wound around both the stator and the rotor to form windings around each member. Applying an electric current to the stator windings produces a magnetic field that rotates at a frequency called the “synchronous frequency”. The rotating magnetic field induces currents in the rotor windings, which in turn, produce another magnetic field.
The two magnetic fields interact by trying to align themselves with each other. This interaction produces a torque, which urges the rotor to rotate. A maximum torque is achieved when the fields are furthest from alignment, and a zero torque is achieved when the fields are aligned (i.e., when the rotor rotates at the synchronous frequency). The difference between the actual rotational frequency of the rotor and the synchronous frequency is called the “slip frequency” and sometimes acts as a factor used in algorithms to control the speed of the motor.
U.S. Patent Application Publication No. 2012/0116617 to Schaffler et al. (Schaffler), published on May 10, 2012, discloses a system and method that provides traction control for wheels on a vehicle in order to provide a substantially similar tractive force exerted by wheels of different sizes on a corresponding surface. Schaffler specifically adjusts the torque applied to wheels of different sizes in order to improve vehicle track adhesion. This means that for wheels of larger size, Schaffler must reduce the torque applied in order to avoid excessive slippage of the larger wheel and reduced track adhesion.
By increasing vehicle track adhesion for all of the wheels on a vehicle, regardless of the size of each wheel, Schaffler does nothing to address the actual cause of excessive slipping or sliding of wheels, which is at least partially a result of the different wheel sizes. The present disclosure is directed towards overcoming one or more of the problems set forth above.